Apparatus for electrical power transmission

ABSTRACT

A device for the transmission of electrical energy includes at least one current converter. Each current converter has phase elements with respective arrangements of circuit elements that comprise at least two switchable power semiconductors each and at least two free-wheeling diodes, each connected in parallel thereto, and energy storing means. The transfer properties in or between power distribution networks are improved with the novel device. The device is provided with means for controlling the current converter in such a manner that the zero crossing, the amplitude and/or the instantaneous values of an alternating current of a transfer network that can be connected to the device and/or the direct current of a direct current line that connects at least one current converter to a direct current source, and/or the direct voltage and the direct current of at least three interconnected current converters can be controlled.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to an apparatus for electrical power transmissionhaving at least one converter, with each converter having phase elementswhich each have an arrangement of switching elements which each compriseat least two power semiconductors which can be switched off and at leasttwo freewheeling diodes, which are respectively connected in parallelwith them, and energy storage means.

Known apparatuses of this generic type have, for example, two converterswhich are connected on the DC voltage side, in order to transmitelectrical power between two electrically isolated, asynchronous or,connected to one another, synchronous AC voltage power supply systems,and to specifically control this transmission. Open-loop control such asthis is necessary since, for example, local overloads or unbalanced loaddistributions can occur in AC voltage power supply systems. The overloadcan then be compensated for by the controlled power transmission. Theseand a series of other apparatuses are referred to as so-called HVDCinstallations and FACTS. Converters in these HVDC installations andFACTS use power semiconductors, such as thyristors, which useline-commutated technology, or power semiconductors which can beswitched off, for example so-called insulated gate bipolar transistors(IGBT), which are used for self-commutated topologies. So-called voltagesourced converters (VSC) with power semiconductors which can be switchedoff require a temporary energy store, generally a capacitor.Arrangements with self-commutated converters and a capacitor as atemporary energy store have the disadvantage that the transmission poweris limited by the size of the capacitor that is used. In the event of afault, an extremely high short-circuit current can lead to destructionof the installation. Until now, only transmission voltages of up toabout ±150 kV and transmission power levels of about 300 to 500Megawatts have therefore been achieved in practice with an arrangementsuch as this.

DE 101 03 031 A1 discloses a converter arrangement for power supplysystem couplings which, instead of an intermediate circuit capacitor,contains distributed capacitors as energy stores, which are contained inindividual switching elements with power semiconductors which can beswitched off, and associated freewheeling diodes.

BRIEF SUMMARY OF THE INVENTION

The object of the present invention is to design an apparatus of thetype mentioned initially which improves the transmission characteristicsin or between power distribution systems.

According to the invention, the object is achieved by means forcontrolling the converter such that the zero phase angle, the amplitudeand/or the instantaneous values of an AC voltage of a transmissionsystem which can be connected to the apparatus, and/or the DC voltageand the direct current on a DC voltage line which connects at least oneof the converters to a DC voltage source, and/or the DC voltage and thedirect current can be controlled by at least three converters which areconnected to one another.

According to the invention, an apparatus having a converter is provided,which has a plurality of individually switchable energy storage means.The control means allow the characteristics of a converter such as thisto be used in the field of power transmission and distribution, and inparticular for power factor correction and for direct-currenttransmission, where the characteristics of a converter such as this areparticularly advantageous. For example, the apparatus according to theinvention is used to improve the stability of the transmission system towhich the apparatus according to the invention can be connected.However, in addition to improving the power supply system stability, itis also possible to optimize the current quality of the powertransmission, in which case the expression current quality in this casecovers the supply reliability and the voltage quality. For this purpose,the voltage produced by the converter is connected in parallel with atransmission line of the transmission system, or is coupled to it inseries, so that the load flow in the transmission system is varied asdesired. Appliances for detection of the AC voltage and of thealternating current are expediently provided in the transmission systemfor parallel connection with accurate phases or for serial coupling ofthe voltage, the measured values from which appliances are supplied to aclosed-loop control unit for the apparatus according to the inventionwhich allows the converter to be controlled on the basis of a comparisonof the measured values with predetermined nominal values. In addition toone or more such closed-loop control units, the control means includeinstruments for detection of measurement variables, software running onthe closed-loop control unit or units, communication devices and thelike. Controllable variables for closed-loop control include, forexample, the AC voltage and/or the alternating current in thetransmission system. The transmission system has one or more phases. Forthe purposes of the claimed invention, an AC voltage should beunderstood as meaning not only a variable at the fundamental frequencybut also a voltage profile which varies in any given manner over time.For example, in the case of essentially sinusoidal AC voltages, the zerophase angle and amplitude of the AC voltage of the transmission systemare preferably controlled. The instantaneous values of the AC voltageare preferably used to control other time profiles of the AC voltage,and these could also be referred to as instantaneous values. Theexpression zero phase angle means the phase difference between the ACvoltage and a reference variable which is dependent on the respectiverequirements applicable to the apparatus according to the invention. Thealternating current in the transmission system at the connection pointis therefore mentioned here just by way of example as a referencevariable.

The converter can be connected to a DC voltage source as well, via a DCvoltage line. By way of example, the DC voltage source is a furtherconverter. Both converters then operate as converters that are connectedto one another on the DC voltage side in a direct-current transmissioninstallation, with the controlled variables being the DC voltage and/orthe direct current on the DC line and/or the AC voltage in thetransmission system. The DC voltage and the direct current are, forexample, detected at each converter and are supplied to a control unitassociated with each of the converters. The closed-loop control makes itpossible to determine the power, and/or the wattless component and/ortheir respective proportions, to be transmitted. Where converters arepositioned at a distance from one another, the nominal parameters aretransmitted by expedient remote data transmission between theconverters. The converters in a direct-current remote transmissioninstallation such as this may advantageously be positioned severalkilometers away from one another.

For the purposes of the invention, a back-to-back link is also claimedfor power transmission and distribution, having at least threeconverters. An apparatus such as this is also referred to as amultiterminal back-to-back link. In this case, by way of example, theconverters are connected to one another directly, that is to say withouta DC line, with direct current and DC voltage being detected at therespective converter or at just one measurement point.

The design and operation of the switching elements are described in DE101 03 031 A1, which is hereby referred to as an integral part of thepresent disclosure. An apparatus such as this has the advantage that thestored energy is distributed between a multiplicity of respectivelysmaller energy storage means, so that the voltage and power restrictionwhen using a single energy storage means, for example a capacitor, isovercome. Furthermore, the distributed energy storage means allow finergraduation of the voltage produced by the converter in comparison toapparatuses with just one common energy store, thus reducing thecomplexity for smoothing and filtering at the apparatus connectingpoint. For example, this considerably simplifies the coupling of theconverter to the transmission system. The invention avoids the need forcomplex magnetic coupling measures, for example by connectingtransformer windings in series. Furthermore, the invention ensuresbetter operational reliability since, if a single switching elementfails, for example as a result of a short circuit, the other switchingelements are still fully operational. The individual switching elementsof a phase element act like controllable voltage sources, and have threepossible states. In a first state, the terminal voltage of the switchingelement is equal to the capacitor voltage. In a second state, theterminal voltage of the switching element is approximately equal tozero, apart from the forward voltage across the power semiconductorwhich can be switched off or the freewheeling diode, with a third statebeing provided for the defect situation.

According to the invention, the apparatus is of modular design. Themodular design is achieved by phase elements which are in turnsubdivided into switching elements. The switching elements are eitheridentical and in particular have identical energy storage means, andtherefore provide the same storage capacity. In contrast to this,however, it is also possible to use combinations with differentcapacitance configurations within the scope of the invention.

In one expedient further development of the invention, the switchingelements of one phase element are connected in series, with an evennumber of switching elements being provided, and a load or power supplysystem connection is arranged centrally on the series circuit formed bythe switching elements. The series connection of a plurality ofswitching elements and an appropriate drive for the individual switchingelements allow an even more finely graduated voltage output. A centralload or power supply system connection means that the switching elementson one side of the series circuit are, for example, in a first state asdescribed further above and the switching elements on the other side arein the second state, likewise as described above, or vice versa. Thesedrives result in maximum voltage values on the phase element. If one ormore switching elements on the respective sides are switched to thesecond state, this results in the voltage being graduated withincrements equal to the voltage on the individual switching elements.

However, phase elements with an odd number of switching elements and/orphase elements with a non-central load or power supply system connectionare likewise possible within the scope of the invention. For example,the individual switching elements are designed for equal or unequalvoltages and are expediently graduated differently, in a binary form orsome other form, thus allowing finer graduation with the same number ofswitching elements than if the design were based on equal voltages.

In one further development, the phase element comprises an arrangementwith two parallel branches, each having an even number of switchingelements connected in series. The connection of two branches inparallel, each having a series circuit formed by switching elements,further increases the fineness of the graduation of the voltage whichcan be generated by the converter.

According to one further development that is expedient in this context,at least two parallel branches are connected to one another by means ofa transformer winding. In contrast to this, at least two parallelbranches are galvanically connected to one another via a parallel branchconnection. The galvanic connection by means of a parallel branchconnection allows a low-cost transformer design, which is used toconnect the apparatus according to the invention to a transmissionsystem and/or to a DC voltage line.

In one expedient development, a plurality of phase elements of aconverter are connected in parallel with one another. In this case, thephase elements form a bridge circuit. The converter acts like aso-called voltage sourced converter (VSC), which is known per se, andcan therefore advantageously be connected to the transmission system inorder to input a controllable polyphase AC voltage for the wattlesscomponent and/or power. In this case, the converter generates apolyphase AC voltage. The control means can be used to selectivelyinfluence the zero phase angle and/or the amplitude of the AC voltage tobe fed into the transmission system, to be precise independently of oneanother. A converter such as this can therefore, for example, also beused as an active filter element instead of or combined with passivefilters, such as RC elements, for active filtering of voltage distortionin the frequency range below and/or above the power supply systemfrequency (subharmonic, super-subharmonic) and/or to compensate forunbalanced voltages. In this case, a voltage input from the converter issuch that the voltage discrepancies from a sinusoidal shape arecancelled out, for example, by negative interference.

Furthermore, a voltage sourced converter such as this can also be usedas a converter for direct-current transmission. The converter thencomprises, for example, three phase elements which are connected inparallel with one another in a known bridge circuit. An arrangement withtwo phase elements connected in parallel also offers a simple option forproviding a converter for direct-current transmission for connection toa transmission system with just one single phase, for example via acoupling transformer, or to a transmission system having a plurality ofphases. The expression direct-current transmission, for the purposes ofthe invention, covers both high-voltage direct-current transmission(HVDC transmission) and medium-voltage direct-current transmission (MVDCtransmission) as well as low-voltage direct-current transmission (LVDCtransmission).

In another embodiment, a plurality of phase elements are connected inseries with one another. The phase elements are advantageously connectedin series with one another with two parallel branches, each having aplurality of switching elements. An arrangement such as this likewiseacts as a voltage sourced converter and, for example, can act as aconverter in a direct-current transmission installation. In this case,the series circuit allows transmission at a higher DC voltage for apredetermined power level, that is to say with a lower current andtherefore with reduced losses.

In one advantageous development, energy storage means are arranged inparallel with the phase elements. Additional energy storage means suchas these are used for further smoothing and stabilization.

In a further refinement, each phase element has at least one impedance,or is connected to another phase element via at least one impedance.Impedances such as these, in the simplest case in the form of coils,advantageously limit any circulating current which may occur between theindividual phase elements for example as a result of voltagefluctuations or unbalanced voltages. In addition, the impedances can bedesigned such that the rate of current rise and/or the current amplitudeare/is limited in the event of malfunctions. In this case, by way ofexample, the impedance is connected in series either with the phaseelement or with individual switching elements of a phase element, or isintegrated in the switching element, for example with an advantageousmodular configuration.

In one preferred embodiment, at least one converter can be connected inparallel with the transmission system. An arrangement such as this isused for so-called parallel compensation for control of the wattlesscomponent and/or power and, for example, carries out dynamic controlfunctions in order to damp out undesirable power fluctuations and/orsubsynchronous resonances and/or subharmonics or super-subharmonics. Theadvantageous further development is also used, for example, for voltagebalancing.

One particularly advantageous feature of the apparatus that has beenfurther developed according to the invention over known parallelcompensation apparatuses is that the series connection of the switchingelements as already described above makes it possible to input an ACvoltage which can be finely graduated into the transmission line, withthe energy to produce the AC voltage being stored in the distributedenergy storage means in the individual switching elements, in contrastto known apparatuses in which a single capacitor is used as the energystore and, because of its size, acts as a limiting element for thetransmission voltage and power of the apparatus. The apparatus accordingto the invention with energy storage means in each switching elementtherefore makes it possible to set the voltage to be fed in more finely.

In a further refinement, at least one converter can be connected inseries with a transmission system. A connection such as this is likewiseused to control the wattless component and/or power in the transmissionsystem, including the already described dynamic control functions, byactive connection and/or inputting of a voltage whose magnitude and/orphase are dynamically variable. The apparatus according to the inventionadvantageously has a plurality of converters, one of which is connectedin parallel with the transmission system, while the other is connectedin series with it. The wattless component and/or the power in thetransmission system are controlled, or else the dynamic controlfunctions as described above are carried out in an improved manner byactively inputting two voltages whose magnitude and/or phase aredynamically variable. By way of example, the transmission system is asingle-phase or a polyphase transmission line.

In another embodiment, the DC voltage source is a second converter. Thesecond converter may in this case act as a rectifier, with the directcurrent being passed via the DC voltage line to the first converter. Thefirst converter or, to be more precise, the inverter, is then used toconvert a DC voltage to an AC voltage. However, the operation of theconverter as a rectifier or inverter is freely variable.

At least part of the DC voltage line is a gas-insulated transmissionline, a cable and/or an overhead line. Combinations of these lines are,of course, also possible within the scope of the invention. Theparticular advantage of a gas-insulated transmission line, GIL, over acable, even in combination with an overhead line, is the capability tocope better with dynamic control and protection functions on the basisof the reduced charge capacitance of the gas-insulated line. Anapparatus according to the invention that has been developed further inthis way is used, for example, for remote direct-current transmission,in order to produce a DC voltage by means of a first rectifier fromsingle-phase or polyphase AC voltages. The DC voltage is transmitted tothe second converter or inverter. The DC voltage transmission inprinciple takes place by means of DC voltage line of any desiredconfiguration.

One advantageous feature when using a converter according to theinvention with three phase elements connected in parallel to one anotheris that no energy storage means need be connected to the DC voltage lineon the DC voltage side since the individual switching elements of thephase elements themselves have energy storage means which are used notonly as energy stores but also for voltage smoothing on the DC voltageside. The use of three phase elements connected in parallel with oneanother in the second converter makes it possible, by means of theswitching elements with energy storage means, to produce a polyphase ACvoltage which can be graduated more finely, for example for feeding intoan AC voltage power supply system that is connected.

DC voltage transmission installations with more than two converters,that is to say so-called multiterminals, are, of course, also feasiblewithin the scope of the invention. In the case of multiterminals, thatis to say direct-current transmission installations with at least threeconverters, the DC voltage line is optional for the purposes of theinvention. In other words, in one variant with at least three converterswhich are linked to one another on the DC voltage side, the inventioncovers not only a remote direct-current transmission installation butalso a back-to-back link.

In one expedient refinement, the first or the second converter is formedby line-commutated power semiconductors. The embodiment of the apparatuswith one converter which, for example, has a bridge circuit formed byline-commutated power semiconductors, for example thyristors or in thesimplest case also diodes instead of the power semiconductors which canbe switched off, makes it possible to reduce the installation costs.

In one expedient further development, the DC voltage line has one or twopoles. Two-pole DC voltage lines allow power levels to be transmitted.Single-pole DC voltage lines which feed the direct current back via theground or, in the case of underwater cable connections through thewater, lead to low-cost apparatuses. Single-phase or two-phasetransmission power supply systems on the alternating-current side of theremote direct-current transmission installation according to theinvention allow connection to special power supply systems, for examplefor railroad supply. DC voltage lines with a plurality of poles are, ofcourse, also possible within the scope of the invention.

In a further embodiment of this further development, the DC voltage lineis formed by an impedance, in the simplest case a coil. By way ofexample, a so-called back-to-back link, which is known per se, can beformed by a coil as a DC voltage line, with the coil carrying outfunctions such as smoothing, current limiting and/or rise gradientlimiting.

In one expedient refinement, a further diode is connected in parallelwith each of the switching elements. A further diode such as this, forexample a pressure-contact diode that is known per se, such as adisk-cell diode or a diode integrated in a pressure-contact electronicsmodule, can bridge a defective switching element, if appropriatelydriven by the control system, if one or more of the switching elementsis or are faulty, thus allowing further operation of the converter. Inthis case, a brief overvoltage is formed deliberately across thedefective switching element by suitably driving the switching elementswhich are still intact, resulting in breakdown of the parallel-connecteddiode and permanently bridging the defective switching element until itis replaced during the next servicing cycle. Furthermore, thefreewheeling diode which is integrated in the power semiconductor canalso have such a bridging function for the switching element in theevent of a fault.

Energy storage means comprise energy stores such as batteries, aflywheel or supercaps and capacitors. The energy stores have aconsiderably higher energy density than capacitors. This has theadvantage that the control of the wattless component and/or power,including the already described dynamic control functions, are stillavailable even in the event of a relatively long voltage dip or failurein the transmission system or in the DC voltage line. The use of energystorage means with a high energy density results in better systemavailability.

At least some of the energy storage means are advantageously capacitors.Capacitors cost less than the currently known energy stores.

In one preferred embodiment, the converter is connected to the DCvoltage line by means of an energy store. When using energy stores witha high energy density, a connection such as this results in bettersystem availability. In this development according to the invention, itis also possible to use as energy stores any of the energy storesmentioned above, with the exception of supercaps. The energy stores areconnected to the DC voltage line in series or in parallel.

The apparatus advantageously forms a direct-current transmissioninstallation and/or a so-called FACTS (flexible AC transmission system)and in this case provides a finely graduated output voltage. A furtheradvantage is that the wattless component and/or power are transmittedwithout complex magnetic coupling. In this case, the apparatus accordingto the invention is advantageously of modular design. It is particularlypreferable to use the apparatus according to the invention fordirect-current transmission and/or for development of a so-called staticsynchronous compensator (STATCOM), of a static synchronous seriescompensator (S3C) or a unified power flow controller (UPFC).

The invention also relates to a system having an apparatus forelectrical power transmission, of the type mentioned above. In this caseas well, the transmission system has one or more phases. In general, thetransmission system is a three-phase line, to which the apparatusaccording to the invention is connected.

The invention will be described in the following text using exemplaryembodiments and with reference to the figures of the drawing, in whichthe same reference symbols refer to components having the same effect,and in which:

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 shows a schematic illustration of one exemplary embodiment of theapparatus according to the invention;

FIG. 2 shows a circuit arrangement of a switching element for theapparatus shown in FIG. 1;

FIG. 3 shows a further exemplary embodiment of a switching element fromFIG. 1;

FIG. 4 shows an exemplary schematic illustration of a converter withphase elements of the apparatus according to the invention connected inseries;

FIG. 5 shows an exemplary schematic illustration of a converter withphase elements of the apparatus according to the invention connected inparallel; and

FIG. 6 shows a further exemplary embodiment of the apparatus accordingto the invention.

DESCRIPTION OF THE INVENTION

FIG. 1 shows, as an apparatus for electrical power transmission, ahigh-voltage direct-current long-distance transmission (HVDC)installation 1 for bidirectional power transmission from a transmissionsystem or AC voltage power supply system 2 to some other AC voltagepower supply system 3. The AC voltage power supply systems 2 and 3 arein this case connected to the HVDC installation via transformers and/orcoils that are not illustrated, or galvanically. The HVDC installation 1has a first converter 4 as a rectifier to convert the AC voltage to a DCvoltage, a transmission cable 5 as a DC voltage line, and a secondconverter 6 as an inverter to convert the DC voltage to an AC voltage.The bipolar transmission cable 5 has two inner conductors 7, 7′ andouter lines 8, 8′, which shield the conductors, are grounded at each oftheir ends, or are protected by other suitable measures, for examplesuppressors. The first converter 4 has three phase elements 10, 11, 12,each of which has a multiplicity of switching elements 10 a . . . 10 i,11 a . . . 11 i and 12 a . . . 12 i arranged in series. In this case,for balancing reasons, each phase element is connected in the center ofthe series circuit formed by the switching elements to a respectivephase of the AC voltage of the AC voltage power supply system 2. Thesecond converter 6 likewise has three phase elements 13, 14, 15, eachhaving an even number of series-connected switching elements 13 a . . .13 i, 14 a . . . 14 i, 15 a . . . 15 i, which each have a connectionfrom one phase of an AC voltage power supply system, in the center ofthe series circuit. At the respective ends of the transmission cable 5,the apparatus also has further circuit arrangements, which are allocated9 and 9′, respectively, comprising capacitors and/or coils and/orresistors and/or suppressors, which are provided for additionalsmoothing of the DC voltage and for transmission stabilization.

Voltage transformers 16, 16′ and current transformers 17, 17′ areprovided respectively for measuring voltage and current, both in the DCvoltage intermediate circuit 5 and in the respective AC voltage powersupply systems 2, 3, with the voltage transformers and currenttransformers on the AC side not being illustrated in the figures, forclarity reasons. The output signals from the voltage transformers 16,16′ and current transformers 17, 17′ correspond to the respectivehigh-voltage component measurement variables to be monitored. Therecorded variables are, in the end, transmitted as measured vales tocontrol units 18, 19 for the apparatus. The signals are sampled in thecontrol units 18, 19 in order to obtain respectively associated samplevalues, and the sample values are digitized to produce digital measuredvalues. The measured digitized measurement currents I_(DC) and/or I_(AC)and the measured digitized measurement voltages U_(DC) and/or U_(AC) arerespectively compared with predetermined nominal values I_(nom) andU_(nom). Means to provide closed-loop control for the apparatus provideopen loop control for the converters 4 and 6 on the basis of open-loopand/or closed-loop control methods.

Further coils, which are not illustrated in the figures, can be arrangedbetween the connections of the respective phase elements 10, 11, 12 aswell as 13, 14, 15, or in each case at the central connection of atleast one of the respective switching elements 10 a . . . 10 i, 11 a . .. 11 i and 12 a . . . 12 i as well as 13 a . . . 13 i, 14 a . . . 14 i,15 a . . . 15 i. The coils limit any possible circulating currentbetween the phase elements.

FIGS. 2 and 3 show equivalent circuit arrangements which are known fromDE 101 03 031 A1 and are used in the apparatus shown in FIG. 1 asswitching elements 10 a . . . 10 i, 11 a . . . 11 i, 12 a . . . 12 i, 13a . . . 13 i, 14 a . . . 14 i, 15 a . . . 15 i. The switching elementshave two connecting terminals 20, 21, two power semiconductors 22, 23,two diodes 24, 25 and a capacitor 26 as the energy storage means. Thepower semiconductors 22 and 23 in the illustrated example are electronicswitches which can be switched off, and in this case IGBTs. However,IGCTs, MOS switching-effect transistors or the like can also be used aspower semiconductors. The operation of the circuit arrangement and theseries connection of a plurality of such switching elements aredescribed in DE 101 03 031 A1 and, by virtue of this reference, are thesubject matter of the present disclosure. The individual switchingelements may be designed for the same or different voltage ranges, and,for example, may also be graduated differently, in a binary form or insome other manner. If required, the additional diode, which is notillustrated in the figures and is used to bridge the switching elementin the event of a fault, is connected to the connecting terminals 20,21.

FIG. 4 shows a further exemplary embodiment of a converter based on aso-called H-circuit for use in an apparatus according to the invention,in which the switching elements 10 a . . . 10 i and 10 a′ . . . 10 i′,11 a . . . 11 i and 11 a′ . . . 11 i′, 12 a . . . 12 i and 12 a′ . . .12 i′, respectively, shown in FIG. 2 are arranged for phase elements 27,28, 29. Each of the phase elements 27, 28, 29 have two parallelbranches, each with series-connected switching elements 10 a . . . 10 iand 10 a′ . . . 10 i′, 11 a . . . 11 i and 11 a′ . . . 11 i′, 12 a . . .12 i and 12 a′ . . . 12 i′. The parallel branches are each connected toone another via two outer connecting lines, which are shown at the topand bottom of FIG. 4, and a central connecting line, with the samenumber of switching elements being connected in series between thecentral and each outer connecting line. The central connecting line ineach case has a phase connection 30, 31, 32 for connection to two phasesof an applied AC voltage. The phase connections 30, 31, 32 areillustrated schematically as connections on the secondaries oftransformers 30, 31, 32, to or from whose primary the respective ACvoltage is applied or tapped off. Capacitors 33, 34, 35 are connected inparallel with the respective phase elements 27, 28, 29, which areconnected in series with one another. When the illustrated arrangementis operated to produce an AC voltage, each phase element feeds an ACvoltage, produced from the DC voltage fed in on the DC voltage side,into one phase of a polyphase AC voltage, by appropriately actuating theindividual switching elements. The capacitors 33, 34, 35 are used foradditional stabilization and smoothing, and are provided onlyoptionally. This arrangement acts on the principle of a voltage sourcedconverter, and generates a three-phase AC voltage from the DC voltageapplied on the DC voltage side or produced by the converter itself. Thearrangement can therefore, of course, also be used as a converter toconvert a three-phase AC voltage to a DC voltage, or vice versa.

FIG. 5 shows a converter with phase elements 27, 28, 29 connected inparallel which allows higher transmission currents to be achieved thanwhen connected in series as shown in FIG. 4. In this embodiment, by wayof example, the phase elements are connected by means of coils 36, 37,38 and 36′, 37′, 38′ to the bipolar direct-current circuit, to which atransmission line, a cable or a GIL, or any desired combination thereof,can be connected.

FIG. 6 schematically illustrates a further exemplary embodimentaccording to the invention of an apparatus for electropower transmission39. The apparatus has a converter 40 which is connected to atransmission line 41 of a transmission system, with the converter 40being connected on the DC voltage side to a capacitor 52 and to anoptional DC voltage source 42. As a transmission system, thetransmission line 41 is part of a power supply system with a loadconnection.

In addition to further means for closed-loop control of the illustratedapparatus according to the invention, an open-loop and closed-loopcontrol unit 43 is used for open-loop and closed-loop control of theconverter with a measured alternating current I_(AC) detected by meansof a current measurement unit 44 and a measured AC voltage U_(AC),obtained by means of a voltage measurement unit 45, being transmitted tothis unit 43, where they are compared with predetermined nominal valuesin order to provide open-loop control dynamically, and matched in phase,for the AC voltage on the transmission line 41, by means of suitableopen-loop control methods. At this point, it should be mentioned onceagain that the expression AC voltage covers any desired voltage timeprofiles applied to the transmission line 41 as a transmission system,and is not restricted to sinusoidal or harmonic voltage profiles.

The converter 40 is connected via an optional coil 46, and a likewiseoptional transformer 47, to the transmission line 41. The converter 40allows control of the wattless component and/or power, or dynamiccontrol functions such as damping of power oscillations and/orsubsynchronous resonances and/or subharmonics and/or super-subharmonicsand/or voltage balancing by actively feeding in a voltage whosemagnitude and/or phase are/is dynamically variable.

The converter 40 has phase elements, which are not illustrated in thefigures, such as the converters 4, 6 illustrated in FIG. 1 and theconverters illustrated in FIGS. 4 and 5. Further assemblies forcompensation 48, 49 with fixed elements and switchable or controllablepower semiconductors 50, 51 are likewise connected to the transmissionline 41. The passive components of the assemblies for compensation 48,49 may comprise any desired combinations of coils, capacitors, resistorsand suppressors and/or individual elements thereof. For example, it isadvantageous to fit the assembly 49 with a resistor, thus providing aswitched or controlled braking resistance for dissipating excess poweron the transmission line 41. Excess power such as this can lead todamaging overvoltages on disconnection of loads or HVDC installationswhich are connected to the transmission line 41.

The assembly 49 advantageously has at least one suppressor. Fitting thissuppressor allows a comparable voltage reduction to be achieved. Theconverter 40 and the assemblies for compensation 48, 49 may be connectedto the polyphase transmission line 41 via the transformer 47, via animpedance or else directly. Compensation and control elements such asthese are also known per se by the expression FACTS. In the case of theapparatus according to the invention described here, the AC voltagegenerated in the converter 40 is actively applied to the transmissionline 41. The converter 40 which is in this case driven as a function ofthe transmission requirements so that the signal that is fed in can bematched in a finely graduated form to the transmission requirements.Mechanical switches such as circuit breakers may also be used, insteadof the power semiconductors 50, 51. In this case, the apparatusaccording to the invention may be in the form of such known FACTS, forexample in the form of a static synchronous compensator (STATCOM), or inthe form of a static synchronous series compensator (S3C) when coupledin series to the transmission line, or in the form of a unified powerflow controller (UPFC) when using a combination of parallel and seriescoupling.

The apparatuses illustrated in FIGS. 1, 4, 5 and 6 may also, within thescope of the invention, be connected to single-phase, two-phase orpolyphase AC power supply systems and transmission lines usingrespective expedient connecting means, in contrast to the illustratedthree-phase AC voltage power supply systems and the three-phasetransmission line 41.

Furthermore, the apparatus shown in FIG. 1, both in the form of thebridge circuit illustrated there and in the variant with convertersforming an H circuit as shown in FIGS. 4, 5, is particularly suitablefor the known HVDC multiterminal operation, that is to say forhigh-voltage direct-current transmission with three or more converters,in which case the converters are connected to one another by means of atransmission line, which is in the form of a cable or a gas-insulatedtransmission line, or else directly forming a so-called back-to-backlink.

The capacitors in the circuit arrangement 9, 9′ shown in FIG. 1, thecapacitors 26 shown in FIGS. 2 and 3, the capacitors 33, 34, 35 shown inFIG. 4 and the capacitors in FIG. 6, including the capacitor 52, may becombined as required with energy stores such as a flywheel, batteries,supercaps or the like, or may be replaced by these energy stores. Forthis purpose, the energy stores are arranged in parallel with, orinstead of, the said capacitors. A spatially concentrated arrangement ina common assembly, for example in the circuit arrangement 9, as well asa distributed arrangement of the energy stores, that is to say a spatialsplitting between different components, are also possible.

LIST OF REFERENCE SYMBOLS

-   1 HVDC installation-   2, 3 AC voltage power supply systems-   4 First converter-   5 Transmission cable-   6 Second converter-   7, 7′ Inner conductor-   8, 8′ Outer conductor-   9, 9′ Circuit arrangement-   10, 11, 12 Phase elements-   10 a . . . 10 i Switching elements-   11 a . . . 11 i Switching elements-   12 a . . . 12 i Switching elements-   10 a′ . . . 10 i′ Switching elements-   11 a′ . . . 11 i′ Switching elements-   12 a′ . . . 12 i′ Switching elements-   13, 14, 15 Phase elements-   13 a . . . 13 i Switching elements-   14 a . . . 14 i Switching elements-   15 a . . . 15 i Switching elements-   16, 16′ Voltage transformer-   17, 17′ Current transformer-   18, 19 Control unit-   20, 21 Connections-   22, 23 Power semiconductor-   24, 25 Diodes-   26 Capacitor-   27, 28, 29 Phase elements-   30, 31, 32 Phase connections-   33, 34, 35 Capacitors-   36, 37, 38 Coils-   36′, 37′, 38′ Coils-   39 System for electrical power transmission-   40 Converter-   41 Transmission line-   42 Energy storage means-   43 Open-loop and closed-loop control unit-   44 Current measurement unit-   45 Voltage measurement unit-   46 Coil-   47 Transformer-   48, 49 Compensation assemblies-   50, 51 Thyristors-   52 Capacitor

1. An apparatus for electrical power transmission, comprising: at leastone converter having phase elements, each of said phase elementsincluding an assembly of switching elements, each of said switchingelements having at least two switchable power semiconductors and atleast two freewheeling diodes, respectively connected in parallel withsaid power semiconductors, and each of said switching elements havingenergy storage means; controller means for closed-loop control of saidat least one converter such that at least one of the following iscontrolled: a zero crossing phase angle, an amplitude, and/orinstantaneous values of an AC voltage of a transmission system to beconnected to the apparatus; and/or a DC voltage and a direct current ona DC voltage line connecting said at least one converter to a DC voltagesource; and/or a DC voltage and a direct current of at least threeconverters that are connected to one another.
 2. The apparatus accordingto claim 1, wherein said switching elements of one said phase elementare connected in series, said switching elements are an even number ofsaid switching elements, and a load connection or power supply system isdisposed centrally on the series circuit formed by said series-connectedswitching elements.
 3. The apparatus according to claim 1, wherein eachof said phase elements has two parallel branches each having an evennumber of switching elements connected in series.
 4. The apparatusaccording to claim 3, which comprises a transformer winding connectingat least said two parallel branches.
 5. The apparatus according to claim3, which comprises a parallel branch connection galvanically connectingat least two parallel branches.
 6. The apparatus according to claim 1,wherein a plurality of said phase elements of a converter are connectedin parallel with one another.
 7. The apparatus according to claim 1,wherein a plurality of said phase elements are connected in series withone another.
 8. The apparatus according to claim 1, which comprisesenergy storage means connected in parallel with said phase elements. 9.The apparatus according to claim 1, wherein each said phase elementincludes at least one impedance, or is connected to a respectively otherphase element via an impedance.
 10. The apparatus according to claim 1,wherein said at least one converter is connected in parallel with thetransmission system or with the DC voltage line.
 11. The apparatusaccording to claim 1, wherein said at least one converter is connectedin series with the transmission system or the DC voltage line.
 12. Theapparatus according to claim 1, wherein said DC voltage source is aconverter.
 13. The apparatus according to claim 1, wherein, at least inplaces, the DC voltage line is at least one line selected from the groupconsisting of a gas-insulated transmission line, a cable, and anoverhead line.
 14. The apparatus according to claim 1, wherein said atleast one converter has line-commutated power semiconductors.
 15. Theapparatus according to claim 1, wherein the DC voltage line has one ortwo poles.
 16. The apparatus according to claim 1, wherein said DCvoltage line is an impedance.
 17. The apparatus according to claim 1,which comprises at least one further diode connected in parallel withsaid switching elements.
 18. The apparatus according to claim 1, whereinsaid transmission system has one or more phases.
 19. The apparatusaccording to claim 1, wherein said energy storage means comprise one ormore capacitors.
 20. The apparatus according to claim 1, wherein said atleast one converter is connected to the DC voltage line by way of anenergy storage device.
 21. The apparatus according to claim 1, whereinsaid at least one converter is one of three converters each connected tothe DC voltage line by way of a respective energy storage device.
 22. Asystem, comprising an apparatus according to claim 1 and a transmissionsystem connected to said apparatus and configured to have at least onephase and to carry an alternating current.
 23. The apparatus accordingto claim 1, wherein said controller means controls the zero crossingphase angle, the amplitude, and/or instantaneous values of the ACvoltage of a transmission system connected to the apparatus.